The transmission of information in space by means of light is an alternative to microwave connections, which is of interest since improved beam bundling and therefore a considerably increased antenna gain are connected with an increase of the carrier frequency. This advantage can be used to reduce the size of the antennas which are to be used, to reduce the corresponding transmission output or to increase the data rate to be transmitted. By means of this it is possible to reduce the weight and the energy consumption, both of which are criteria which are of decisive importance for satellite systems to be operated in space.
In optical communications both the direct reception which, although uncomplicated, is susceptible to background light, and the highly sensitive superimposed reception, which therefore is particularly suited for space applications, are offered. The gain in sensitivity with superimposed reception in contrast to direct reception, however, results in a considerably more elaborate realization, and furthermore makes greater demands on the components used.
A very narrow divergence angle is connected with the high antenna gain of optical antennas or telescopes, for which reason a very exact alignment of the antennas in respect to each other is necessary. In this case the beam regulation systems must be able to make connections free of disruptions possible, in spite of systematic and stochastic movements of the satellites.
As described in Wittig, M. et al. "In-Orbit Measurement of Microaccelerations of ESA's Communications Satellite OLYMPUS", SPIE Proceedings, vol. 1218 (1990), pp. 205 to 213, it is possible to model and interpret the stochastic movements as two Gaussian-distributed angular fluctuations for a satellite, which have an output density spectrum S.sub..phi. represented below: ##EQU1## wherein the frequency of the angular fluctuations is identified by f.
A part of these angular fluctuations can be controlled by a beam regulating system, and a standard deviation of an uncompensated error signal of each component is obtained (see Hyden, W. et al., "Wide-Band Precision Two Axis Beam Steerer Tracking Servo Design and Test Results", SPIE Proceedings, vol. 1866 (1993), pp. 271 to 279): ##EQU2## wherein the standard deviation of the uncompensated error signal of the two components is identified by .sigma..sub..phi..sbsb.x.sub., Rest, .sigma..sub..phi..sbsb.y.sub., Rest, and the interference signal transmission function of the beam regulating system by G(f). The uncompensated angular fluctuations are connected with a fluctuation of the detected output and result in an increase of the error probability of the compensating system.
It is possible in many cases to describe the interference signal transmission function as a first order high-pass filter with a limit frequency f.sub.g : ##EQU3## Thus, the angular fluctuations are the better suppressed the higher the bandwidth of the beam regulating system.
However, a central problem in connection with beam regulation lies in finding a low-noise error signal suitable for a broad-band regulation.
Usually distinctions between the following concepts are made in optical superimposition systems:
In connection with obtaining an error signal by means of a direct reception, the received light is divided into two partial beams by means of a beam splitter, for example a semipermeable mirror. In this case one portion is usually coupled into a glass fiber, the light of a local laser is superimposed on it in a fiber coupler and is supplied to the coherent receiver of the communication system.
A second portion (partial beam) is supplied to a position detector, a CCD camera or a so-called four quadrant photodiode, and an error signal, in particular two spatial error signals, are generated by means of a suitable evaluation of the quadrants. If, for example, a sum signal is formed from the first two detector quadrants and is subtracted from the sum signal which is formed from the other two quadrants, an azimuthal error signal is obtained. By means of a cyclical exchange of the quadrants it is furthermore possible to obtain an elevation error signal (U.S. Pat. No. 5,030,004).
In this connection a combination of different detectors is also possible and known. The splitting of the beam into two portions can be omitted if, besides the communication laser, an additional laser of a different wavelength (BEACON) is used for beam regulation in the transmitter.
However, obtaining an error signal becomes problematic if the receiving telescope can also catch background light. In this case the signal-to-noise ratio of the error signal and therefore the noise suppression is bad. The portion of the received light possibly split off for the beam regulation is not available to the communications branch. The transmitting output required for a defined error probability is increased by this.
The adjustment of the detectors of the beam regulation system in connection with the communication system furthermore must meet the highest requirements. Because of the long signal processing times in the case of CCD cameras, or because of the bad signal-to-noise ratio because of background light, the attainable bandwidth of the noise suppression is clearly less than 1 kHz as a rule.
With the so-called Nutator principle, the directional characteristic of a receiving telescope is periodically changed, for example by means of a circular movement of the glass fiber of the receiver. Alternatively to this it is possible to deflect the received beam by means of movable lenses, mirrors or also acusto-optically or electro-optically.
If in this case the receiving telescope is not optimally aligned, the light output detected by the receiver fluctuates. It is possible by means of suitable demodulation of this detected output to generate an error signal for beam regulation. Here, too, a CCD camera is mostly used in addition for the acquisition of the received beam.
In connection with the Nutator principle sketched above it is disadvantageous that with such methods the maximally attainable bandwidth of the noise suppression is approximately one tenth of the frequency of the circular movement superimposed on the received beam. For optical satellite communication the rotational frequency in this case must usually be greater than 10 kHz and, in case of a mechanical deflection of the received beam or of the glass fiber of the receiver makes the greatest demands of the components used. For this reason the use of such components in space appears to be not without problems.
This disadvantage is avoided, for example with an electro-optical or acusto-optical beam deflection, wherein as a rule this partially results in considerable optical losses.
In connection with the electro-optical or acusto-optical systems, the error signal for spatial beam regulation is mostly obtained by means of a synchronous demodulation of the detected optical output. In the process it is necessary, for example, to consider and control the temperature-dependent phase shift of the components which are involved in the beam regulation. A remaining error, which is unavoidable in actual use, results in a systematic loss in sensitivity, the same as the circular movement of the receiving characteristic.
A method for obtaining error signals for a spatial beam regulation of an optical superimposition receiver is known from European patent application EP-A2-0 642 236, in which an arrangement of silicon photodiodes is employed for the coherent reception of a data signal and for the direct reception of error signals.